1. Field of the Invention
The present invention relates to a method and a device for generating voltage peaks in electrostatic precipitators, in particular those electrostatic precipitators which are used for purification in power stations and similar installations, the gas discharges of which contain dust particles.
2. Description of Related Art
One way of separating dust particles from a gas is to use an electrostatic dust precipitator, sometimes also referred to as an ESP or an electrostatic precipitator. Examples of common areas of application of electrostatic precipitators are coal-fired power stations, cement factories and refuse incineration.
As is clear from the name, an electrostatic precipitator makes use of electrostatic forces to separate dust particles from a gas. Broadly speaking, this is effected in the following manner: the gas is led into a chamber which contains vertical metal curtains which divide the chamber into a number of parallel gas passages. Arranged centrally in each passage is a frame with electrodes arranged in it, which can often consist of wires. All the frames are interconnected, and form a continuous framework. The entire framework is suspended in supporting insulators, which results in the framework being electrically insulated from the other parts of the precipitator.
A high-voltage rectifier which generates rectified voltage, for example in the form of pulses, is connected between the framework and earth, as a result of which an electric field is obtained between the wires in the framework and the metal curtains. This electric field causes the dust particles in the gas to move towards the plate curtains, and to adhere to these. By shaking or knocking the plate curtains, an accumulated mass of dust is freed and falls down, under its own weight, into a dust container intended for it. It can be shown that the quantity of dust removed from the gas depends on the growth derivative of the voltage pulses; the higher the growth derivative, the more dust can be separated from the gas. This is especially noticeable in the case of dust with high resistivity.
In connection with electrostatic precipitators, it is of course desirable to achieve as high a degree of separation as possible, at the same time as the price of the separation arrangement is to be kept as low as possible.
Previously known arrangements for generating rectified voltage pulses in electrostatic precipitators can, broadly speaking, be divided into two categories. The first category generates rectified voltage pulses of which the growth derivative is in step with that of the mains voltage, which results in a relatively inexpensive arrangement which, however, on account of a low growth derivative, has a relatively low degree of cleaning, particularly for dust with high resistivity.
The second category of previously known arrangements for generating rectified voltage pulses in electrostatic precipitators generates rectified voltage pulses with a high growth derivative by means of oscillating circuits. The energy in the pulses is, in other words, fed back to the arrangement. These arrangements achieve a high degree of dust separation for dust with high resistivity as well, but are relatively expensive.
In other words, two main categories of arrangements for generating rectified voltage pulses in electrostatic precipitators have existed previously, one type achieving low cost, and the other type achieving a high degree of separation.
U.S. Pat. No. 4,648,887 shows a method for controlling electrostatic precipitators. The method shown in this document comprises generating voltage pulse-trains to an electrostatic precipitator by means of current pulses, with the pulses of each voltage pulse-train comprising a number of sub-pulses. A drawback of this method is that each voltage sub-pulse corresponds on a one-to-one basis to a current pulse. The current pulses come from rectified AC from an ordinary AC-source. The time between the current pulses and thus between the voltage sub-pulses is thus determined by the frequency of the AC-source used, and cannot be varied.
The problem solved by the present invention is that of providing an electrostatic precipitator which allows a high degree of cleaning, or dust separation, at a lower cost than has previously been possible, especially for dust with high resistivity.
This problem is solved by means of a method for generating individual voltage peaks in an electrostatic precipitator via generation of current pulses, comprising rectifying AC from a power supply into DC, with monitoring and controlling of said rectification so that each individual voltage peak in the precipitator is built up by a group of pulses of DC current, which pulses are supplied to the precipitator. The build-up of the voltage peaks in the precipitator is monitored and controlled, and each current pulse group is discontinued when their corresponding voltage peak in the precipitator has reached a desired value.
Preferably, the method according to the invention further comprises rectifying and smoothing the AC from the power supply into DC in a first step, and converting and transforming the DC from said first step into high-frequency AC, and also rectifying said high-frequency AC into corresponding DC-pulses.
In a preferred embodiment of the method according to the invention, the current pulses in each group are generated with such an amplitude and frequency that the voltage peaks increase with a derivative which exceeds 30 kV/ms.
The problem is also solved by means of a device for generating individual voltage peaks in an electrostatic precipitator via generation of current pulses, said device comprising first rectifying means for rectifying AC from a power supply into DC, means for monitoring and controlling said first rectifying means so that each voltage peak in the precipitator is built up by a group of pulses of DC-current, which pulses are supplied to the precipitator, and means for monitoring and controlling the build-up of the voltage peaks in the precipitator. The device also comprises means for discontinuing each current pulse group when their corresponding voltage peak in the precipitator has reached a desired value.
Preferably, the first rectifying means in the device according to the invention additionally comprises means for rectifying and smoothing the AC from the power supply into DC in a first step, means for converting and transforming the DC from said first step into high-frequency AC, and means for rectifying said high-frequency AC into corresponding DC-pulses.
In addition, the device according to the invention may additionally comprise means for varying the number of DC-pulses in each DC-pulse group to the precipitator, to reach the desired value of each voltage peak.
In a preferred embodiment, the device according to the invention is provided with means for generating voltage peaks which increase with a derivative which exceeds 30 kV/ms.